Medical Physics- skeleton bones

Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
Medical Physics
{ Physics of the Skeleton}
Most of the skeleton is made of bones held together by ligaments. The
bone is mostly made of hard bone tissue, tough cover, blood vessels and
marrows.
Functions of the bones :
Bones has six functions in the body :
1. Support :- bones and muscles of the legs support the body.
2. Locomotion :- bone joints permit movement of one bone with
respect to another.
3. Protection :- The skull protect the brain, eyes and ears . The ribs
protect heart and lungs .
4. Storge of chemicals:- Ca storage in the bone which is released
when it needed.
5. Nourishment:- Teeth :
6. Sound transmission :- Ossicles in the middle ear ( the smallest bone
in the body).
The bone is a living tissue and has blood supply as well as nerves, the
cells that maintain the bone in a healthy condition , they make up
about 2% of the volume of the bone which called "osteocytes"
Bone Remodeling : A continuous process of destroying old bone and
building new bone.
There are two types of cells in bone remodeling :
1. Osteoblasts : cells specialized in bone building.
2. Osteoclasts : cells specialized in bone destroying.
Each day the osteoclast destroy bone containing about 0.5 gm of Ca ,
and Osteoblast builds a new bone using nearly the same amount of Ca.
While the body young and growing , activity over 35 40 years old
the process is reversed causing a gradual decrease in bone mass , that
continuous until death and this is called Osteoprosis (Porous bones).
This appeared to be more in women than in men specially in spine and
hip.
Note : the bones have about 1000gm of Ca so we have a new skeleton
about every six years .
Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
What is Bone Made Of ?
Bone consist of two different materials plus water :
1. Collagen :- the major organic fraction is about 40% of the weight
of solid bone and 60% of it's volume .
2. Bone Mineral :- Inorganic material , which is about 60% of the
weight and 40% of volume of solid bone.
When the bone mineral removed from the bone the reminder is collagen ,
the collagen reminder is quite flexible like rubber and can be bent into
loops. When the collagen is removed from the bone , the bone reminder is
very fragile and can be crushed with the fingers.
..Collagen is apparently produced by the Osteoblasts cells, mineral is
then formed on the collagen to produce bone. Bone collagen is not the
same as the collagen found in many other parts of the body such as the
skin. Bone mineral is believed to be made up of calcium hydroxyapatite
..
Studies using X-Ray show that the bone mineral constructed from very
small rod shaped crystal with diameter of 20 70 A° and length of
about 50 100 A°( 1A°= 10-10 m) Because of the small size of the
crystal, bone mineral has very large surface area. In a typical adult , it has
a surface area of over 4×105 m2 .
Note : A round each crystal is alayer of water containing in solution
many chemicals needed by the body . the large area of exposed bone
mineral crystal permit the bones to intract rapidly with chemicals in the
blood and other fluids.
Compositions of of compact bone :-
Elements H C N O Mg P S Ca others
% Comp. 3.4 15.5 4 44 0.2 10.2 0.3 22.2 0.2
Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
Note: Because of the large percentage of calcium (Ca) in the bones and
because calcium has a much heaver nucleus than most elements of the
body, the bones absorbs X-Ray much better than the surrounding soft
tissue and this the reason that X-Ray show bones so well .
How strong are your body ?
There are about 200 bones in the body . The bones are shaped and
constructed into five piles according to their functions to withstand forces
exert on it.
1. Plate like bones:- The shoulder blade ( scapula) and some bones of
the skull.
2. Long hollow bones :- Bones of the arms , Legs and fingers.
3. Cylindrical bones :- Bones forms the spine ( vertebrae).
4. Irregular bones :- The wrist and ankle bones.
5. Ribs :- Which do not belong in any of the other piles.
If one cut some of the bones apart , he can find it composed of one or
a combination of two quite different types of bone.
1.Solid or compact bone
2.Spongy or cancellous bone(Trabecular bone) .
What are the advantages of trabecular bone over compact bone?
The trabecular bone gives bone the strength necessary with less
materials than compact bone.
The trabecular are relatively flexible and bone can absorb more energy
when large forces are involved such as in walking, running and
jumping .
Note: - The density of the bone is about 1.9 gm/cm2 which stays
constant through life.
If a bone is placed under tension or compression , it's length (L) is
changed and the strain ( L / L ) increases linearly at first , i.e :
proportional to stress ( F/ A) where (F) is the applied forces and(A) is
the cross sectional area of the bone.
As the force increase the length increase rapidly and the bone breaks
at the stress of about 120 N/mm2 .
Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
Stressn
160
140
120
100
80
60
40
20
0
0 0.005 0.01 0.015 0.02
Strain
Stress F /A
Y
Strain L /A
Y : young modulus
(Y) for bones = 1.8×1010 N/m2
To calculate the change in length ( L) for a given force F , the above
equation can be written as :
LF
L = _____
AY
Example:-
Assume a leg has 0.6m shaft of bone with an average cross sectional
area of 1.5cm2 , What is the amount of shortening when all of the body
weight of 350N is supported on the leg :
LF (0.6m)(3.5 102N )
L Y 1.8 1010 N / m 2
AY (1.5 10 4m2)(1.8 1010N / m2)
0.77 10 4m
0.77mm
Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
Note:- Healthy bone is able to withstand a compression stress of about
170 N/mm2 before it fractures.
The property of bone to withstand a large force for a short period
of time without breaking . While some force over along period may
fracture it, called Viscoelasticity.
There is a good evidence that local electrical fields may play a role ,
When bone is bent it generates an electrical charge on it's surface. It
has been suggested that this phenomenon (Piezoelectricity ) may be
the physical stimulus for bone growth and repair.
Lubrication of Bone Joints
There are two major diseases may effect the joints : -
1. Rheumatoid Arthritis ,Which results in over production of
synovial fluid in the joint and causes swollen joint.
2. Osteoarthrosis , a disease of the joint itself .
The main components of joint are :
1. The synovial membranes encases the joint and retain the
lubricating synovial fluid.
2. Articular cartilage : a smooth rubbery material that is
attached to the solid bone. The fat in the cartilage helps to
reduce the coefficient of fraction(0.1 for healthy joints).
The lubrication properties of a fluid depend on it's viscosity , thin oil is
less viscous and better lubricant than thick oil. The viscosity of synovial
fluid decrease under the large shear stresses found in the joint.
Note:- Synovial fluid is a good lubricant due to the presence of
hyaluronic acid and mucopoly saccharides.
Studying the bone Invivo ( In living body).
Studying the body Invitro( used on excised bone sample).
Ass. Lecturer. Zaid Kamil Alnewaini University of Kufa
M.Sc. Medical Physics College of Dentistry
Measurement of Bone Mineral in The Body
The strength of the bone depends on the mass of bone mineral present.
Bone mineral mass decrease very slowly (1-2% per year) so they very
precise technique was needed to show the changes are :-
1. The using image to measure the amount of bone mineral.
2. Using photo absorptiometry ( it s a develop X-ray image).
3. Invivo activation.
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